The invention relates to semiconductor devices and components and, specifically, to methods for forming gate dielectrics for semiconductor devices and components.
Dielectric materials are a key aspect to the performance of semiconductor devices. As devices become smaller, and the need for higher performance becomes greater, the thickness of the dielectric layers in semiconductor devices is decreasing. At the same time, the need for dielectric materials with a dielectric constant greater than that of the most common dielectric material, SiO2, is increasing. Also, as the thickness of the dielectric layer in semiconductor devices decreases, the need for materials that do not leak charge even when the layer of the dielectric material is very thin (e.g. less than 100 xc3x85) is increasing.
However, not all dielectric materials form acceptable, thin dielectric layers for use in semiconductor devices and components. Semiconductor devices have certain performance requirements such as efficiency, power of operation, etc. The properties of the layer of dielectric material directly effect device performance. For example, if the thin dielectric layer allows too much current to pass through it (this unwanted current is referred to as leakage current), then the resulting device or components will not meet the desired performance requirement. Since the leakage current through the gate dielectric of a MOSFET (metal-oxide-semiconductor-field-effect-transistor) indicates the insulation properties (resistance and reliability) of the dielectric, a gate dielectric layer through which the leakage current is too high indicates that the resistance and reliability of the dielectric layer is too low.
The interface state density between the dielectric layer and the underlying semiconductor interface also affects device performance. The interface state density degrades the current drive (current across the channel) and the reliability of MOSFETs and MIS(metal-insulator-semiconductor) FETs. Thus, if the interface state density is too high, then the resulting device or component will not meet the desired performance requirement.
Consequently dielectric materials that form thin dielectric layers with acceptable leakage characteristics and other properties are sought.
The present invention is directed to a process for fabricating a semiconductor device. In the process, a single crystal semiconductor substrate is provided. The substrate typically has structures such as device channel regions, and field oxide regions formed therein. The gate dielectric is then formed on the substrate. The gate dielectric is either a metal oxide, a metal silicate or both. A metal silicate material has the general structure (MO2)x(SiO2)y, wherein M is a metal, Si is silicon and O is oxygen. The relative mole fractions of these elements in the metal silicate are represented by x and y. Therefore, the sum of x and y is equal to 1. It is advantageous if the mole fraction of the metal oxide (x) is about 0.05 to about 0.8. It is advantageous if the mole fraction of the metal oxide is about 0.05 to about 0.5. The present invention contemplates metal oxides and metal silicates that contain one or more metals.
The dielectric materials of the present invention are advantageous because they are thermodynamically stable on silicon even when exposed to the high temperatures (i.e. over 800xc2x0 C.) that are intrinsic to MOSFEIT fabrication. A stable material will maintain structural and chemical xe2x80x9cintegrityxe2x80x9d because it does not react with the substrate. A structurally stable material does not undergo a phase change (e.g. from amorphous to polycrystalline) after formation. For example, Ta2O5 is not suitable because it reacts with silicon at 700xc2x0 C. to form a tantalum-silicon-oxygen interface layer. Such an interface layer is undesirable because its thickness and composition are not controllable. In the process of the present invention, the undesirable interface layer is not formed, and the metal oxide or metal silicate formed on the substrate is stable.
The gate dielectric is formed on a prepared silicon substrate surface. The prepared surface has either a very thin (i.e. less than about 1.5 nm) oxide or silicon oxynitride layer formed thereon, is a hydrogen-terminated surface, or, advantageously, a clean silicon surface. The cleaned surfaces are formed using techniques well known to one skilled in the art.
The dielectric layer is formed on the prepared surface of the silicon substrate. The deposition conditions are selected to favor the formation of the metal silicate or metal oxide over the formation of silicon dioxide (SiO2). Specifically, gaseous precursors that favor the formation of the metal silicate over silicon oxide are selected.
The gate dielectric layer is formed by chemical vapor deposition (CVD). In the CVD process, a first precursor (referred to as the inorganic precursor) is provided as the source for the metal component of the metal oxide or metal silicate. The first precursor has at least one metal-containing compound. If the dielectric layer is a metal silicate, the inorganic precursor has a silicon-containing compound in addition to a metal-containing compound. The silicon-containing compound is the source for silicon in the metal silicate. Thus, when the dielectric material is a metal silicate, it is contemplated that the first precursor is either one compound that is the source for both metal and silicon or two compounds (one being a metal source and the other being a silicon source). A second precursor (referred to as the organic precursor) is provided as the source for the oxygen in the dielectric layer.
The metal is any metal or combination of metals suited for forming a metal oxide or metal silicate dielectric layer. It is advantageous if the dielectric material has a dielectric constant of at least about 10. Examples of metals that form metal oxides and metal silicates with sufficiently high dielectric constants include zirconium, hafnium, lanthanum, yttrium, tantalum, aluminum, cerium and titanium. The metal-containing compound is selected to provide reaction kinetics that favor the formation of metal oxide or metal silicate on the silicon surface. Specifically, it is advantageous if the temperature at which the metal-containing compound decomposes is higher than the deposition temperature. Candidate metal compounds have decomposition temperatures at least above 200xc2x0 C. It is advantageous if the precursors have a decomposition temperature above the deposition temperature so that the reaction by which the dielectric is formed is adequately controlled. Examples of suitable metal compounds include metal tetrachlorides (e.g. hafnium tetrachloride) and metal alkoxides (e.g. zirconium t-butoxide). For the metal alkoxides, it is advantageous if the alkyl moiety has no more than six carbon atoms to ensure that the metal alkoxide has a suitably high volatility.
The silicon containing compound, if present, is a silicon precursor such as tetraethyl orthosilicate (TEOS), silane or dichloro silane. Such precursors are well known to one skilled in the art and not discussed in detail herein. It is advantageous if the decomposition temperature of the silicon-containing compound is also above the deposition temperature.
As previously noted, the organic precursor is the source for oxygen. The organic precursor serves to catalyze a reaction with the inorganic precursor and with the surface of the silicon substrate to form the dielectric layer. However, the reaction does not favor the formation of SiO2. Thus, the present invention affords an advantage over prior art processes where SiO2 is formed along with the metal oxide or metal silicate. This is because, in prior art processes, oxygen species such as O2 favor the formation of SiO2. Examples of suitable organic precursors include alkyl oxides, alkyl phosphine oxides, alkyl sulfoxides and heterocyclic oxides. The alkyl moieties have 1-3 carbon atoms (e.g. methyl, ethyl and propyl groups). The heterocyclic oxides include oxanorbomene and oxanorbomadiene.
The organic precursor is also selected based on the range of temperatures over which it decomposes (i.e., its reaction temperature) and its ability to provide a byproduct that does not react with silicon. For example, dimethyl sulfoxide reacts with silicon and dimethylsulfide is the side-product of that reaction. Trimethylphosphine oxide reacts with silicon and trimethylphosphine is the side-product of that reaction. Oxanorbomadiene reacts with silicon and benzene is the side-product of that reaction. Oxanorbomene reacts with silicon and cyclohexadiene is the side-product. Ethylene oxide reacts with the silicon surface to form oxidized silicon and ethylene.
The temperature at which the metal or silicon is deposited defines the temperature range at which the organic precursor should decompose. Thus, an organic precursor with a decomposition temperature that is at or below the deposition temperature is selected.
The prepared substrate is then placed in a CVD tool for depositing the dielectric material. As previously noted, in order to ensure that a dielectric material with the desired composition and properties is obtained, it is advantageous if the wafer is heated to a temperature that is below the temperature at which the inorganic and organic precursors decompose. Although applicants do not wish to be held to a particular theory, applicant""s believe that the desired reaction environment is found at a temperature below the decomposition temperature of the precursors. Above the decomposition temperature, there is a greater likelihood of side reactions that do not produce the desired dielectric material. It is advantageous if the wafer is heated to a temperature that is at least 50xc2x0 C. below the lower decomposition temperature of the two precursors. It is particularly advantageous if the wafer is heated to a temperature that is at least 100xc2x0 C. below the lower decomposition temperature of the two precursors. The temperature to which the wafer is heated thus depends upon the decomposition temperature (also referred to as the cracking temperature) of the two precursors.
The organic precursor is flowed into contact with the substrate. It is advantageous if the flow rate and partial pressure of the organic precursor is at least about five times the flow rate and partial pressure, respectively, of the inorganic precursor. As previously noted, the temperature of the substrate is below the decomposition temperature of the organic precursor. It is advantageous but not required for the organic precursor to be flowed into contact with the substrate first. The inorganic precursor is then flowed into contact with the substrate. The temperature and pressure are selected to ensure that the inorganic precursor molecules are contacting the substrate surface in the vapor phase but not depositing metal or oxygen thereon. In this regard, bare silicon surfaces are advantageous for trapping the inorganic precursors, followed by hydrogen-terminated silicon surfaces. It is also contemplated that the precursors will be pulsed in. That is, a pulse of organic precursor, followed by a pulse of inorganic precursor, following by a pulse of organic precursor, etc.
Under the selected conditions, the organic precursor catalyzes a reaction between the inorganic precursor molecules on the substrate. The selected conditions depend upon the decomposition temperature of the inorganic and organic precursors. In this regard, the deposition temperature is selected to facilitate the reaction between the two precursors to form the desired dielectric. The reaction continues until the desired dielectric thickness is obtained. The thickness of the dielectric film is monitored using conventional techniques for monitoring film thickness during the CVD process. When the desired thickness is obtained, the process is ceased by stopping the flow of the reactant gases into the chamber. The substrate is then allowed to cool and is then removed from the tool.